Methods and apparatus for removing contamination from lithographic tool

ABSTRACT

Embodiments described herein provide a method for cleaning contamination from sensors in a lithography tool without requiring recalibrating the lithography tool. More particularly, embodiments described herein teach cleaning the sensors using hydrogen radicals for a short period while the performance drifting is still above the drift tolerance. After a cleaning process described herein, the lithography tool can resume production without recalibration.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 16/655,164 filed Oct. 16, 2019, which is adivisional application of U.S. patent application Ser. No. 15/898,813filed Feb. 19, 2018, which claims priority to U.S. Provisional PatentApplication 62/565,791, filed on Sep. 29, 2017, the entire disclosuresof both of which are incorporated herein by reference.

BACKGROUND

In semiconductor manufacturing, lithographic apparatus is used to applypatterns onto a substrate by selectively exposing a photoresist layer onthe substrate to a radiation source. The size and/or density of featuresin the patterns may be limited by the wavelength of the radiation sourceused by the lithographic apparatus. Extreme ultraviolet (EUV)lithography, which uses extreme ultraviolet (EUV) radiation or softx-ray, i.e. radiation with wavelength shorter than 130 nm, has becomeone of the lithography methods for forming smaller semiconductordevices.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1A is a schematic graph showing an extreme ultraviolet (EUV)illumination tool according to some embodiments.

FIG. 1B is a partial sectional view of the EUV illumination tool showingsensors on a substrate stage according to some embodiments.

FIGS. 1C-1D are partial sectional views of the EUV illumination toolduring a contamination removal process according to some embodiments.

FIGS. 2A-2C are schematic plots of sensor measurements in the EUVillumination tool according to some embodiments.

FIG. 3 is a schematic flow chart of a method for patterningsemiconductor substrates according to some embodiments.

FIG. 4 is a schematic plot showing sensor readings during EUVlithography with contamination removal sections according to someembodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

FIG. 1A is a schematic graph showing an EUV illumination tool 100according to some embodiments. The EUV illumination tool 100 includes aradiation source 102 configured to supply EUV radiation 108. Theradiation source 102 may be a laser produced plasma source. A hot plasmamay be produced from a gas or vapor, for example Xe gas, Li vapor, or Snvapor, using a laser light to emit radiation in the EUV range. Theradiation source 102 may produce a radiation having a wavelength in therange from about 5 nm to about 20 nm, for example, a wavelength of about13.5 nm, or a wavelength from about 6.7 nm or about 6.8 nm. In someembodiments, the EUV illumination tool 100 is used to producesemiconductor devices at a 5 nm technology node.

The radiation source 102 emits the EUV radiation 108 to a condenser 110.The condenser 110 includes surfaces 112A, 112B configured to focus theEUV radiation 108 and a reflector 114 configured to reflect the EUVradiation 108 towards a reticle 104. The reticle 104 may be secured to amask stage 106. The reticle 104 has a pattern surface 120 having apattern to be transferred to a workpiece.

In this disclosure, the terms of reticle, mask, and photomask are usedto refer to the same item. In the EUV illumination tool 100, the reticle104 is a reflective mask. The reticle 104 may include a substrate,multiple reflective layers formed on the substrate, and a patternedlayer formed over the multiple reflective layers. The substrate may be asubstrate of a low thermal expansion material or fused quartz. A lowthermal expansion material may include titanium dioxide (TiO₂) dopedwith silicon dioxide (SiO₂). The multiple reflective layers may includea plurality of film pairs, such as molybdenum-silicon (Mo/Si) filmpairs, molybdenum-beryllium (Mo/Be) film pairs, or other suitablematerial pairs that are highly reflective to EUV lights. The patternedlayer may be an absorption layer patterned with a pattern to define alayer of an integrated circuit. The absorption layer may be a tantalumboron nitride (TaBN) layer. Alternatively, the patterned layer may be apatterned reflective layer, thereby forming an EUV phase shift mask.

The pattern surface 120 of the reticle 104 reflects the radiation 108from the condenser 110 towards a projection optics module 118. Theprojection optics module 118 includes a series of mirrors, such asmirrors 116A-116D. The mirrors 116A-116D function as lenses to reducethe size of the pattern carried by the EUV radiation 108.

During operation, the projection optics module 118 projects the EUVradiation 108 towards a substrate 122 disposed on a substrate stage 124.The substrate 122 may be a semiconductor substrate on which integratedcircuit devices are to be formed. The substrate 122 may be a bulksemiconductor substrate (e.g., a wafer), a silicon on insulator (SOI)substrate, or the like. Materials of the substrate 122 can includesilicon, silicon germanium, germanium, gallium arsenide, polysilicon,silicon oxide, carbon doped silicon oxide, silicon nitride, glass, andsapphire. It is contemplated that the substrate 122 is not limited toany particular size or shape. Therefore, the substrate 122 may be acircular substrate having a 200 mm diameter, a 300 mm diameter or otherdiameters, such as 450 mm, among others. The substrate 122 may also beany polygonal, square, rectangular, curved or otherwise non-circularworkpiece.

The substrate 122 has a photoresist layer formed thereon. The EUVradiation 108 is incident on the radiation sensitive photoresist layertransferring the pattern carried in the EUV radiation 108 to thephotoresist layer.

The photoresist layer may include any suitable photoresist materialdesigned for the EUV wavelength. In some embodiments, the photoresistlayer may include a chemically amplified resist (CAR). A CAR may beformulated by adding an organic polymer, a photo acid generator, and aquencher species together. Alternatively or additionally, thephotoresist layer may include a metal based photoresist. For example,the photoresist layer may include a metal-oxide resist on top of asacrificial carbon layer, such as spin-on-carbon. The photoresist layermay also be a tri-layer mask having a bottom layer, a middle layer, anda top layer. The bottom layer may be a carbon organic layer. The middlelayer may be a silicon-containing carbon layer used to help pattern thebottom layer. The top layer may be any suitable photoresist materialdesigned for exposure to the EUV wavelength.

The substrate stage 124 may include an electrostatic chuck 126configured to secure the substrate 122 thereon during operation. Theelectrostatic chuck 126 may be formed from a rigid material having a lowcoefficient of thermal conductivity. The electrostatic chuck 126 may beconnected to a number of actuators configured to the move electrostaticchuck in a number of degrees of freedom to focus the EUV radiation 108on the substrate 122 and/or to align the pattern on the reticle 104 witha target portion on the substrate 122. In some embodiments, thesubstrate stage 124 may be configured to move the substrate 122 in sixdegrees of freedom—X, Y, Z, Rx, Ry, and Rz using any number ofactuators, such as six actuators.

A number of sensors 130 a, 130 b, 130 c, 130 d (collectively sensors130) may be disposed on a top surface 128 of the electrostatic chuck126. The sensors 130 are positioned to be proximate to the substrate 122during operation, for example near the edge of the substrate 122. Thesensors 130 may be fixedly mounted on the electrostatic chuck 124 andmay be used to evaluate and/or optimize imaging performance of the EUVillumination tool 100. One or more sensors 130 may include an upperplate that is transparent to radiation, such as radiation in the EUVwavelength, or may include a pattern of transparent portions and opaqueportions. The upper plate may be positioned to receive radiation fromthe EUV radiation 108. The received radiation may be directed to one ormore transducers in the sensor 130. The sensor 130 may include anoptical element, such as a fiber optic plate or micro lens array, thatis suitable to direct or focus the received radiation to the transducer.The transducer may be a device suitable to convert radiation to anelectric signal, such as a photodiode, a CCD camera, or a CMOS camera.The output of the transducer may be used to control, calibrate, oroptimize the operation of the EUV illumination tool 100.

In some embodiments, the sensors 130 a, 130 b may be transmission imagesensors (TIS). A TIS sensor is used to measure the position of aprojected aerial image of a mask pattern on the reticle 104 at thesubstrate level. The projected image at the substrate level may be aline pattern with a line having comparable wavelength to the wavelengthof the radiation 108. The measurement of the TIS sensors 130 a, 130 bmay be used to measure the position of the mask with respect to thesubstrate stage 124 in six degrees of freedom, e.g., three degrees offreedom in translation and three degrees of freedom in rotation.Additionally, magnification and scaling of the projected pattern mayalso be measured by the TIS sensors 130 a, 130 b. The TIS sensors 130 a,130 b are capable of measuring pattern positions, influences ofillumination settings, such as sigma, numerical aperture of lens. TheTIS sensors 130 a, 130 b may be used to align the reticle 104 with thesubstrate 122, focus the EUV radiation 108 to a target region on thesubstrate 122, measure performance of the EUV illumination tool 100,and/or measure optical properties, such as pupil shape, coma, sphericalaberration, astigmatism, and field curvature. Even though, two TISsensors 130 a, 130 b are shown in FIG. 1A, less or more TIS sensors maybe included according to the design of the illumination tool 100.

In some embodiments, the sensor 130 c may be a spot sensor configured tomeasure a dose of EUV radiation at the substrate level. The measured EUVradiation by the spot sensor 130 c at the substrate level can be used tocalculate the EUV radiation absorbed by mirrors in the path of the EUVradiation 108 for compensating the effects of EUV radiation loss, whichmay improve optical performance of the EUV illumination tool 100.

In some embodiments, the sensor 130 d is an integrated lensinterferometer at scanner (ILIAS). An ILIAS sensor is an interferometricwave front measurement device that performs static measurement on lensaberrations up to high order. The ILIAS sensor 130 d may be used tomeasure wavefront errors in the EUV radiation 108.

It should be noted that other sensors may be included in the substratestage 124 to achieve target functions. Different sensors may be combinedinto one sensor to achieve multiple functions. For example a TIS sensormay be combined to with an ILIAS sensor to measure both projected aerialimages and wavefront errors.

In some embodiments, the EUV illumination tool 100 includes a housing140. The housing 140 defines an inner volume 142. A vacuum pump 144 maybe connected to the housing 140 to establish a vacuum environment in theinner volume 142. The substrate stage 124 is disposed in the innervolume 142 so that the EUV lithographic process may be performed in avacuum state.

Other components of the EUV illumination tool 100, such as the condenser110, the mask stage 106, and the projection optics module 118, may bedisposed in the housing 140 or in individual housings.

In some embodiments, the EUV illumination tool 100 includes a cleaningmodule 132 configured to remove contamination from inner surfaces of theillumination tool 100, such as outer surfaces of the sensors 130. Amongother contaminations, the cleaning module 132 may be used to removedepositions on optical surfaces resulting from dissociation of thephotoresist layers, lubricants, and uses of pumps.

The cleaning module 132 may include a radical generator 134 configuredto provide radicals into the inner volume 142 to remove contaminations.The radical generator 134 may generate radicals using a hot filament,oscillating field electrode, a magnetron RF generator. For example, theradical generator 134 may be a hydrogen radical generator configured togenerate hydrogen radicals using one or more hot filaments. Hydrogenradicals may react with contaminations, such as carbon deposits releasedby photoresist, to form volatile hydrocarbons, such as methane (CH₄).Volatile hydrocarbons may be removed from the inner volume 142 by thevacuum pump 144.

In some embodiments, the radical generator 134 is disposed in the innervolume 142. The cleaning module 132 may include an actuator assembly 136configured to direct the radical generator 134 to a component to becleaned, such as sensors 130. In some embodiments, the actuator assembly136 is a robot.

The EUV illumination tool 100 further includes a controller 138configured to perform a cleaning procedure according to someembodiments. The controller 138 may be configured to monitor one or moreparameters of the EUV illumination tool 100 to determine whether acleaning process should be started. In some embodiments, the controller138 is connected to the sensors 130 to monitor one or more sensormeasurements. Based on the monitored sensor measurements, the controller138 will determine whether the sensors 130 need cleaning. The controller138 sends commands to the radical generator 134 to perform a cleaningprocess.

FIG. 1B is a partial sectional view of the EUV illumination tool 100showing contamination on the sensors 130 mounted on the substrate stage124. Photoresists, such as organic photoresists, may outgas hydrocarbonsin the vacuum environment in the EUV illumination tool 100. Theoutgassed hydrocarbons may disassociate to carbon under the radiation,such as the EUV radiation 146 used to pattern the substrate 122.Overtime, carbon deposits may accumulate on an outer surface 150 of thesensors 130 forming a contamination 148 and causing the sensors 130 tolose accuracy. The outer surface 150 may be configured to receiveradiation to complete measurements by the sensor 130. In addition tocarbon, the EUV illumination tool 100 may include other elementsreleased into the vacuum during EUV lithographic process, such as metalsoutgassed from metal oxide based photoresist, zinc released from solderor the EUV source, or zinc originated from trace element in stainlesssteel.

Contaminations 148 on the sensors 130 cause performance of the EUVillumination tool 100 to drift overtime. For example, carbon depositedon the upper surface of the TIS sensors 130 a, 130 b may cause the EUVillumination tool 100 to have a focus drift. In a continuously operatingEUV illumination tool, TIS sensors on the substrate stage may have afocus drift at about 10 nm per month due to carbon deposit on theradiation receiving surface. Carbon deposited on the upper surface ofthe spot sensor 130 c may cause the EUV illumination tool 100 to have alot-to-lot critical dimension variation. In a continuously operating EUVillumination tool, spot sensors on the substrate stage may have alot-to-lot critical dimension (CD LtL) variation of about 0.5 nm due tocarbon deposit on the radiation receiving surface. Carbon deposited onthe upper surface of the ILIAS sensor 130 d may cause the EUVillumination tool 100 to have a proximity drift. In a continuouslyoperating EUV illumination tool, ILIAS sensors on the substrate stagemay have a proximity drift at about lnm every six month due to carbondeposit on the radiation receiving surface.

After performing a number of exposures, depending on energy level usedduring exposure, lot size, and/or source power, an EUV illumination toolmay have contamination built up on sensors and need cleaning. Typically,a continuously operating EUV illumination tool similar to the EUVillumination tool 100 is typically shut down for about 48 hours at leastevery six months to clean off the carbon deposits. The cleaning processtypically takes about 24 hours to remove the carbon deposit and another24 hours to recalibrate the EUV illumination tool 100. Carbon deposit onthe sensors may be removed manually, or using a hydrogen radicalgenerator. The calibration of the EUV illumination tool 100 includesperforming pilot runs with batches of substrates. The long shut downtime, materials, and labor involved increase cost of ownership of theEUV lithographic tools.

Embodiments as described herein provide a method for cleaningcontamination from the sensors without requiring recalibration of theEUV illumination tool 100. More particularly, embodiments describedherein permit cleaning the sensors using hydrogen radicals for a shortperiod while the performance drifting is still above the drift tolerancewithout recalibrating the EUV illumination tool 100 after the cleaning.

The radical generator 134 may be used to deliver radicals, such ashydrogen radicals, towards the outer surface 150 of the sensors 130 toremove the contamination 148. FIG. 1C is a partial sectional view of theEUV illumination tool 100 showing contamination removal from the sensors130 using the radical generator 134 according to some embodiments.

The radical generator 134 may include a housing 152 defining an innervolume 154. The housing 152 includes an inlet 156 for receiving aprecursor gas from a gas source. The housing 152 also includes an outlet158 configured to output radicals. One or more filaments 160 may bearranged in the inner volume 154 between the inlet 156 and the outlet158. The filaments 160 may be a tungsten (W) or tantalum (Ta) wire orcoil which can be heated by electricity. The filaments 160 may be heatedto a desired temperature to dissociate bonds in molecules in theprecursor gas to generate radicals. In some embodiments, the radicalgenerator 134 further includes cooling pipes 162 wound around thehousing 152. A cooling fluid, such as water, may circulate in thecooling pipes 162 to prevent the radical generator 134 from overheatingthe environment.

In some embodiments, a hydrogen containing gas may be provided to theinner volume 154 of the radical generator 152 through the inlet 154. Thehydrogen containing gas may be a gas mixture including hydrogenmolecules, such as hydrogen (H₂), hydrogen deuteride (HD), deuterium(D₂), hydrogen triteride (HT), and tritium (T₂). A mixture of hydrogencontaining gas and noble gases, such as helium (He) may be supplied tothe inlet 154. In some embodiments, the filament 160 may be heated tobetween about 1500° C. to about 3000° C. to generate hydrogen radicalsfrom hydrogen molecules in the hydrogen containing gas.

During operation, the radical generator 134 may be moved adjacent to thesensor 130 to be cleaned, for example using the actuator assembly 136attached to the radical generator 134. In some embodiments, the outlet158 may be pointed toward the outer surface 150 of the sensor 130. Insome embodiments, the outlet 158 of the radical generator 134 may bepositioned at a distance of less than about 5 cm to the outer surface150. Positioning the radical generator 134 close to the sensors 130prevents hydrogen radicals from diffusing away, which may improveefficiency.

Additionally, positioning the radical generator 134 close to the sensors130 can also reduce hydrogen radicals reacting with other components inthe EUV illumination tool 100, such as reflective surfaces on themirrors. Contaminations of other type, such as tin, zinc, metals, ormetal oxides, may react with hydrogen radicals to form metal hydridewhich may redeposit on optical elements damaging the optical elements.

As shown in FIG. 1C, hydrogen radicals are dispatched from the radicalgenerator 134 toward the sensor 130 to remove the contamination 148. Asshown in FIG. 1D, the hydrogen radicals react with carbon in thecontamination 148 to form methane, which is volatile, thus removing thehydrogen contamination from the sensor 130.

In some embodiments, the cleaning process shown in FIGS. 1C-1D may be amini cleaning session carried out during operation before thecontamination causes the amount of performance drift that would triggera normal cleaning session, which is usually preprogrammed in the EUVillumination tools by the manufacturer. In some embodiments, the minicleaning session lasts only a fraction of downtime of a normal cleaningprocess. In another embodiment, the mini cleaning session is a localcleaning session that only involves one sensor or a portion of thesensors and is performed only for a fraction of normal cleaning time.Alternatively, the mini cleaning session may be performed using adifferent recipe than the normal cleaning process, for example, with ahigher or lower partial pressure of the hydrogen radicals or with avariation of precursors in addition to shortened cleaning time.

According to embodiments described herein, the mini cleaning session maybe triggered when one or more sensor measurements reach a thresholdvalue. In the normal cleaning process, the sensors 130 are cleaned whenthe measurements of the sensors 130 are affected by the contaminationsthereon causing the performance drift of the EUV illumination tool 100to reach or close to reaching the performance drift tolerance for theprocess. Unlike the normal cleaning process, the mini cleaning sessionaccording to some embodiments is performed when a sensor measurementindicates some degree of contamination accumulation but before theperformance drift has occurred or the amount of performance driftwarrants a normal cleaning process. Unlike in the normal cleaningprocess, recalibration or pilot runs may be obviated performed aftereach mini cleaning session, which may further reduce downtime andeliminate cost of raw materials used during recalibration and pilotruns.

FIGS. 2A-2C are schematic plots of monitored sensor measurements in theEUV illumination tool 100 according to some embodiments. FIG. 2A shows acurve 202 of relative radiation intensity i_(TIS) received by a TISsensor, such as the TIS sensors 130 a, 130 b, over time. The relativeradiation intensity i_(TIS) is a ratio of absolute radiation intensityI_(TIS) received by a TIS sensor positioned at the substrate stage overabsolute source radiation intensity I_(source) dispatched by a radiationsource, such as the EUV source 102. The absolute radiation intensityI_(TIS) may be obtained through transducers in the TIS sensor, such asthe TIS sensors 130 a, 130 b.

The absolute source radiation intensity I_(source) may be obtained fromsensors in the EUV radiation source 102. Alternatively or additionally,the absolute radiation intensity of the EUV radiation beam measured inother components upstream to the TIS sensors, such as the condenser 110,the mask stage 106, and/or the projection optics module 118, may be usedas the absolute source radiation intensity I_(source) in calculating therelative radiation intensity i_(TIS).

A decreased relative radiation intensity i_(TIS) indicates an intensityloss caused by contamination on the radiation receiving outer surface ofthe TIS sensor, such as the outer surface 150. FIG. 2A illustrates howthe relative radiation intensity i_(TIS) received by the TIS sensor ismonitored and used as a trigger to start a mini cleaning sessionaccording to some embodiments. As shown in FIG. 2A, the relativeradiation intensity i_(TIS) decreases over time due to accumulation ofcontamination, such as the carbon deposits. When the relative radiationintensity i_(TIS) reaches a threshold value i_(TIS0), a mini cleaningsession may be started. In some embodiments, the threshold valuei_(TIS0) may be above about 0.6. For example, the threshold valuei_(TIS0) may be in a range from about 0.6 to about 0.9. In someembodiments, the threshold value i_(TIS0) may be about 0.9. In someembodiment, the threshold value i_(TIS0) may vary depending on thesource of the absolute source radiation intensity I_(source). Forexample, a lower threshold value i_(TIS0) may be selected when theabsolute source radiation intensity I_(source) is obtained from acomponent further upstream from the TIS sensor to be cleaned consideringintensity loss in a longer reflection path.

FIG. 2B shows a curve 204 of relative radiation intensity i_(spot)received by a spot sensor, such as the spot sensors 130 c, over time.The relative radiation intensity i_(spot) is a ratio of absoluteradiation intensity I_(spot) received by a spot sensor positioned at thesubstrate stage over absolute source radiation intensity I_(source)dispatched by a radiation source, such as the EUV source 102. Theabsolute radiation intensity I_(spot) may be obtained throughtransducers in the spot sensor, such as the spot sensor 130 c.

A decreased relative radiation intensity i_(spot) indicates an intensityloss caused by contaminations on the radiation receiving outer surfaceof the spot sensor, such as the outer surface 150. FIG. 2B illustrateshow the relative radiation intensity I_(TIS) received by the spot sensoris monitored and used as a trigger to start a mini cleaning sessionaccording to embodiments of the present disclosure. As shown in FIG. 2B,the relative radiation intensity i_(spot) decreases over time due toaccumulation of contamination, such as the carbon deposits. When therelative radiation intensity i_(spot) reaches a threshold value i_(spot)a mini cleaning session may be started. In some embodiments, thethreshold value i_(spot0) may be above about 0.6. For example, thethreshold value i_(spot0) may be in a range from about 0.6 to about 0.9.In some embodiments, the threshold value i_(spot0) may be about 0.9. Insome embodiment, the threshold value i_(spot0) may vary depending on thesource of the absolute source radiation intensity I_(source). Forexample, a lower threshold value i_(spot0) may be selected when theabsolute source radiation intensity I_(source) is obtained from acomponent further upstream from the spot sensor to be cleanedconsidering the longer reflection path.

FIG. 2C shows a curve 206 of aberrations a measured by an ILIAS sensor,such as the ILIAS sensor 130 d, over time. The aberrations a may bemeasured by a transducer in the ILIAS sensor receiving the EUV radiationin the EUV illumination system 100. An increase in aberrations aindicates a change in optical properties of the ILIAS sensor caused bycontaminations on the radiation receiving outer surface of the ILIASsensor, such as the outer surface 150.

FIG. 2C illustrates how the aberrations a measured by the ILIAS sensoris monitored and used as a trigger to start a mini cleaning sessionaccording to some embodiments. As shown in FIG. 2C, the measuredaberrations a increase over time due to accumulation of contamination,such as the carbon deposits. When the measured aberration a reaches athreshold value a₀, a mini cleaning session may be started. In someembodiments, the threshold value a₀ may be in a range from about 0.2 nmto about 0.3 nm.

FIG. 3 is a schematic flow chart of a method 300 for patterningsemiconductor substrates according to some embodiments. The method 300relates to patterning semiconductor substrates by an EUV lithographicprocess. The method 300 may be performed using the EUV illumination tool100 described above. In some embodiment, the method 300 is started aftera normal cleaning process. Operations 310, 320, 330 are performed inloops without interruptions of any additional normal cleaning process orrecalibration process.

Operation 310 in the method 300 includes patterning semiconductorsubstrates using an EUV tool, such as the EUV illumination tool 100,while monitoring sensor measurements in the EUV tool. Semiconductorsubstrates having a photoresist layer formed thereon may be patternedconsecutively in the EUV tool. One or more sensors, such as a TISsensor, a spot sensor, and an ILIAS sensor, may be mounted on asubstrate stage in the EUV tool and used to control, monitor, evaluate,and/or optimize imaging performance of the EUV tool.

One or more sensor measurements from at least one of the sensors may bemonitored during the patterning process. In some embodiments, the one ormore sensor measurement indicates an amount of contamination accumulatedon an outer surface of the corresponding sensor. The outer surface isconfigured to transmit radiation to a transducer in the correspondingsensor. In some embodiments, the outer surface is a surface of a coverplate that is entirely transparent or has one or more portionstransparent to radiation in the wavelength used for patterning thesubstrates.

In some embodiments, a measurement of each sensor having a viewport forreceiving radiation may be monitored. Alternatively or additionally,only measurement by one or more representative sensors may be monitored.In some embodiments, the one or more sensor measurements include arelative radiation intensity i_(TIS) received by a TIS sensor, such asthe TIS sensors 130 a, 130 b. The one or more sensor measurements mayalso include relative radiation intensity i_(spot) received by a spotsensor, such as the spot sensors 130 c. The one or more sensormeasurements may also include aberrations a measured by an ILIAS sensor,such as the ILIAS sensor 130 d.

In some embodiments, the one or more sensors in the EUV tool are at apristine condition when operation 310 starts. The pristine condition maybe the condition immediately after a normal cleaning process.

In operation 320, the sensor measurement(s) may be analyzed to determinewhether to start a mini cleaning session. The determination may beperformed in a system controller, such as the controller 138 in the EUVillumination tool 100. Whether to start a mini cleaning session may bedetermined based on if at least one of the sensor measurements hasreached a corresponding threshold value.

For each TIS sensor, a relative radiation intensity i_(TIS) may bemonitored and the corresponding threshold value i_(TOS0) may be aboveabout 0.6, such as in a range from about 0.6 to about 0.9. In someembodiments, the threshold value i_(TIS0) may be about 0.9.

For each spot sensor, a relative radiation intensity i_(spot) may bemonitored and the corresponding threshold value i_(spot0) may be aboveabout 0.6, such as in a range from about 0.6 to about 0.9. In someembodiments, the threshold value i_(spot0) may be about 0.9.

For each ILIAS sensor, a measured aberration a may be monitored. In someembodiments, the threshold value a₀ for the measured aberration a may bebetween about 0.2 nm to about 0.3 nm.

In some embodiments, a mini cleaning session may be started when onesensor measurement reaches the corresponding threshold value.Alternatively, a mini cleaning session may be started when two or moresensor measurements reach the corresponding threshold values. In otherembodiments, a mini cleaning session may be started when a predeterminedportion of the sensor measurements, such as 50% of monitored sensormeasurements, reach the corresponding threshold value.

If it is determined in operation 320 that the conditions to start a minicleaning session have not been met, patterning in operation 310 cancontinue. If it is determined in operation 320 that the conditions tostart a mini cleaning session have been met, the operation of the EUVillumination tool of operation 310 will be stopped (e.g., afterpatterning one or more substrate and before patterning another one ormore substrate) and operation 330 started to perform a mini cleaningsession.

The mini cleaning session may last only a fraction of the downtime of anormal cleaning process. In some embodiments, a mini cleaning sessionmay only take about 2% to 5% of the downtime used in a normal cleaningprocess. For example, the mini cleaning session may take between about30 minutes to about 60 minutes downtime.

In some embodiments, the duration of a mini cleaning session is selectedto return the sensor conditions to or near to the pristine conditions,which are the conditions at the beginning of operation 310. In someembodiments, the duration of the mini cleaning session is a fixedaccording to empirical results. In some embodiments, the duration of themini cleaning session may be selected from a look up table of a seriesof monitored sensor conditions and a series of cleaning durations. Thelook up table may be obtained by experiments. A longer mini cleaningsession corresponds to more contamination formed over the sensors, andvice versa. In some embodiments, the mini cleaning session may beperformed using the same cleaning device and cleaning recipe used in thenormal cleaning process except that the mini cleaning session isperformed at a fraction of time. For example, the mini cleaning sessionmay be performed using a factory cleaning recipe supplied by the EUVtool manufacturer with an adjustment for cleaning time.

Alternatively or additionally, the mini cleaning session may be a localcleaning session that only involves one or a portion of the sensors andis performed only for a fraction of normal cleaning time. Alternativelyor additionally, the mini cleaning session may be performed using adifferent recipe than the normal cleaning process, for example with ahigher or lower partial pressure of the hydrogen radicals or with avariation of precursors in addition to shortened cleaning time.

In some embodiments, the mini cleaning session includes generatinghydrogen radicals using a hydrogen radical generator and delivering thehydrogen radicals towards the sensors to be cleaned. Contaminationsaccumulated over the sensors may react with the hydrogen radicals toform volatile hydrocarbons, such as methane, which can be removed fromthe inner volume of the EUV tool.

In some embodiments, the mini cleaning session includes moving thehydrogen radical generator toward the one or more sensors to be cleaned,generating and supplying hydrogen radicals to the sensors to removecontamination therefrom.

After the mini cleaning session in operation 330, the method 300continues with the operation of the EUV illumination tool to perform thepatterning process in operation 310 without going through a calibrationprocess typically following a normal cleaning process of the EUV tool.

During traditional operation of the EUV tool, sensor measurements and/ora substrate being processed are monitored to determine whether a normalcleaning process is needed. The EUV tool is shut down for a normalcleaning when the sensor measurements reach threshold values orperformance data indicates a substantial performance drift. Under theconditions triggering a normal cleaning process, performance of thesensors is also drifted far enough and the sensors need to berecalibrated after the normal cleaning process. The sensor calibrationprocess adds additional down time to the EUV tool. As illustrated inFIG. 4, when performing mini cleaning sessions as described herein, thesensors performance does not drift far enough between cleaning sessionsto trigger a recalibration process. As a result, the overall performancecan be improved and the down time can be reduced.

FIG. 4 includes plots showing sensor measurements and performanceresults of an exemplary EUV lithography process with contaminationremoval sessions according to some embodiments. The exemplary EUVlithography process may be performed using a method similar to themethod 300 described above. The EUV lithography process may be performedusing an EUV illumination tool similar to the EUV illumination tool 100.The EUV illumination tool includes at least a TIS sensor, a spot sensor,and an ILIAS sensor mounted on a substrate stage configured to monitor,evaluate, control, and/or optimize the image performance of the EUVillumination tool.

During the exemplary EUV lithography process of FIG. 4, condition of theTIS sensor may be monitored through a relative radiation intensityi_(TIS) calculated from measurement by the TIS sensor. Curves 402 a, 402b, and 402 c are schematic plots of the relative radiation intensityi_(TIS). Condition of the spot sensor may be monitored through arelative radiation intensity i_(spot) calculated from measurements ofthe spot sensor. Curves 404 a, 404 b, and 404 c are schematic plots ofthe relative radiation intensity i_(spot). Conditions of the ILIASsensor may be monitored using aberrations a measured by the ILIASsensor. Curves 406 a, 406 b, and 406 c are schematic plots of theaberration a.

Line segments 420 includes mini cleaning sessions performed during theexemplary lithography process. A first mini cleaning session wasperformed at time t₁ when the relative radiation intensity i_(TIS)calculated from measurement by the TIS sensor reaches the thresholdvalue. After the first mini cleaning session, the patterning processresumes at time t′₁ and all sensor measurements have improved from themeasurements at time t₁. A second mini cleaning session was performed attime t₂ when the relative radiation intensity i_(spot) calculated frommeasurement by the spot sensor reaches the threshold value. After thesecond mini cleaning session, the patterning process resumes at time t′₂and all sensor measurements have improved from the measurements at timet₂. A third mini cleaning session was performed at time t₃ when theaberration measurement by the ILIAS sensor reaches the threshold value.

In some embodiments, the time gap between t₁ and t₂ and time gap betweent₂ and t₃ is about one week. The time gaps between t₁ and between t₂ andt′₂, and between t₃ and t′₃ are in a range from about 30 minutes toabout 60 minutes. The sum of downtime of mini cleaning sessions over asix month period under these circumstances would be between 13 hours to26 hours, a reduction of more than 50% of downtime compared to thenormal cleaning process with about 48 hours of downtime.

Curves 408, 410, 412 are schematic plot of focus drift, CD LtLvariation, and proximity drift of the EUV tool without mini cleaningsessions. Curves 414, 416, 418 are schematic plot of focus drift, CD LtLvariation, and proximity drift of the EUV tool the mini cleaningsessions. As shown by curves 408, 412, 414, 418 the focus drift andproximity drift increase over time when no mini cleaning sessions areperformed while the focus drift and proximity drift are almostundetectable with the mini cleaning sessions. Through the comparison ofcurves 410 and 416, the CD LtL variation reduces about 60% by performingthe mini cleaning session.

Even though the above description relates to removing contamination fromsensors in an EUV illumination tool, embodiments of the presentdisclosure can be used to remove contaminations from other components inan EUV illumination tool, such as from the mask stage and the optics.Even though EUV lithographic tools and processes are described above,embodiments of the present disclosure may be used in any lithographictools operating in a vacuum condition to remove contamination frominternal components, such as sensors.

Embodiments of the present disclosure includes a method for lithographicprocess with mini cleaning sessions for remove contamination fromsensors in a lithographic tool, such as an EUV tools. The mini cleaningsessions provide several advantages. For example, by performing minicleaning sessions, downtime is reduced by illuminating the recalibrationor pilot run process following normal cleaning process. Further, the sumof downtime of mini cleaning sessions is shorter than the downtime ofthe normal cleaning process. Additionally, the mini cleaning sessionsensure that the sensors are at a higher level of cleanness compared tousing the normal cleaning process, thus, improve overall imagingperformance of the lithographic tool.

Some embodiments provide a method. The method includes monitoring asensor measurement indicative of an amount of contamination formed on aradiation receiving surface over a sensor in a lithography tool, andperforming a cleaning session in the lithography tool when the sensormeasurement reaches a threshold value. The cleaning session comprisesproviding radicals to the radiation receiving surface. After thecleaning session, the sensor is operable to provide the sensormeasurement without being recalibrated

Some embodiments provide a method. The method includes patterningsemiconductor substrates sequentially using a lithography tool whilemonitoring a sensor measurement of a sensor in the lithography tool,performing a cleaning session in the lithography tool when the sensormeasurement reaches a threshold value, and resuming patterningsemiconductor substrates sequentially using the lithography tool. Duringresuming patterning the semiconductor substrates, the sensor is operableto provide the sensor measurement without being recalibrated after thecleaning session.

Some embodiments provide a lithography tool. The tool includes a housingdefining an inner volume, a substrate stage configured to secure andmove a substrate in the inner volume, a sensor mounted on the substratestage, wherein the sensor includes an outer surface configured toreceive radiation, a radical generator configured to deliver radicals tothe inner volume, and a controller communicatively coupled to the sensorand the radical generator. The controller is configured to monitor asensor measurement from the sensor indicative of an amount ofcontamination formed on the outer surface of the sensor, and cause theradical generator to perform a cleaning session when the sensormeasurement reaches a threshold value. The cleaning session includesproviding radicals from the radical generator to the outer surface ofthe sensor, and the sensor is operable to provide the sensor measurementafter the cleaning without recalibration.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. An apparatus for removing contamination from alithographic tool, the apparatus comprising: a sensor that measures anamount of contamination formed on a radiation receiving surface over thesensor in a lithography tool; and a cleaning module that performs acleaning session in the lithography tool when the sensor measures arelative radiation intensity reaches a threshold value, wherein thecleaning module includes a radical generator configured to provideradicals to the radiation receiving surface, wherein after the cleaningsession, the sensor provides the sensor measurement without beingrecalibrated, wherein the sensor is a spot sensor mounted on a substratestage in the lithography tool, and the sensor measurement includes therelative radiation intensity defined by a ratio of a radiation intensitymeasured by the spot sensor through the radiation receiving surface overa radiation intensity dispatched from a radiation source.
 2. Theapparatus of claim 1, wherein the threshold value of the relativeradiation intensity is in a range from 0.6 to 0.9.
 3. The apparatus ofclaim 1, wherein the spot sensor measures a lot-to-lot criticaldimension variation (CD LtL) caused by carbon deposited on an uppersurface of the spot sensor.
 4. The apparatus of claim 3, wherein thethreshold value of the lot-to-lot critical dimension variation (CD LtL)is less than 0.5 nm.
 5. The apparatus of claim 1, wherein the sensorfurther includes an ILIAS (integrated lens interferometer at scanner)sensor mounted on a substrate stage in the lithography tool, and thesensor measurement includes an aberration of a radiation received by theILIAS sensor through the radiation receiving surface measured by theILIAS sensor.
 6. The apparatus of claim 5, wherein the threshold valueof the aberration of the radiation received by the ILIAS sensor is in arange from 0.2 nm to 0.3 nm.
 7. The apparatus of claim 1, wherein thecleaning session comprises generating hydrogen radicals using aninternal radical generator.
 8. The apparatus of claim 7, wherein thecleaning session further comprises moving the hydrogen radical generatortowards the sensor before generating hydrogen radicals.
 9. A cleaningdevice comprising: semiconductor substrates sequentially patterning by alithography tool while monitoring a sensor measurement of a sensor inthe lithography tool; and the cleaning device that performs a cleaningsession in the lithography tool when the sensor measurement including arelative radiation intensity reaches a threshold value; and thesemiconductor substrates resuming patterning by the lithography tool,wherein during resuming patterning the semiconductor substrates, thesensor provides the sensor measurement without being recalibrated afterthe cleaning session, wherein the sensor is a TIS (transmission imagesensor) sensor mounted on a substrate stage in the lithography tool, andthe sensor measurement includes the relative radiation intensitymeasured by the TIS sensor.
 10. The cleaning tool of claim 9, whereinthe threshold value of the relative radiation intensity is in a rangefrom 0.6 to 0.9.
 11. The cleaning tool of claim 9, wherein the sensormeasurement is indicative of an amount of contamination formed on aradiation receiving surface over the sensor.
 12. The cleaning tool ofclaim 9, wherein the sensor measurement further includes at least oneselected from the group of: a first relative radiation intensity definedby ratio of a radiation intensity measured by a TIS sensor through aradiation receiving surface of the TIS sensor over a radiation intensitydispatched from a radiation source, a second relative radiationintensity defined by a ratio of a radiation intensity measured by a spotsensor 130 c through a radiation receiving surface of the spot sensor130 c over the radiation intensity dispatched from the radiation source,and an aberration of a radiation received by an ILIAS sensor through aradiation receiving surface of the ILIAS sensor.
 13. The cleaning toolof claim 12, wherein the sensor measurement includes at least two sensormeasurements selected from the group of the first relative radiationintensity, the second relative radiation intensity, and the aberration,and wherein the cleaning session is performed when at least one of thesensor measurements reaches a corresponding threshold value.
 14. Thecleaning tool of claim 12, wherein the sensor measurement includes thefirst relative radiation intensity, the second relative radiationintensity, and the aberration, and wherein the cleaning session isperformed when all of the sensor measurements reach correspondingthreshold values.
 15. The cleaning tool of claim 12, wherein thecleaning session is performed for a duration in a range from about 30minutes to about 60 minutes.
 16. The cleaning tool of claim 9, whereinperforming the cleaning session comprises generating hydrogen radicalsusing an internal radical generator.
 17. The cleaning tool of claim 9,wherein performing the cleaning session comprises performing apre-programmed cleaning process for a duration that is between about 2%and about 5% of a pre-defined duration of a full cleaning session. 18.The cleaning tool of claim 9, wherein performing the cleaning sessioncomprises moving the hydrogen radical generator towards the sensorbefore generating hydrogen radicals.
 19. A lithography tool, comprising:a housing defining an inner volume; a substrate stage configured tosecure and move a substrate in the inner volume; a sensor mounted on thesubstrate stage, wherein the sensor includes an outer surface configuredto receive radiation; a radical generator configured to deliver radicalsto the inner volume; and a controller communicatively coupled to thesensor and the radical generator, the controller being configured to:monitor a sensor measurement from the sensor indicative of an amount ofcontamination formed on the outer surface of the sensor; and cause theradical generator to perform a cleaning session when the sensormeasurement reaches a threshold value, the cleaning session comprisingproviding radicals from the radical generator to the outer surface ofthe sensor, the sensor being operable to provide the sensor measurementafter the cleaning without recalibration, wherein the sensor is selectedfrom a group consisting of a TIS (transmission image sensor) sensor, aspot sensor, and an ILIAS (integrated lens interferometer at scanner)sensor mounted on a substrate stage in the lithography tool.
 20. Thelithography tool of claim 19, wherein: the sensor measurement includesat least one of: a relative radiation intensity defined by ratio of aradiation intensity measured by the TIS sensor through an outer surfaceof the TIS sensor over a radiation intensity dispatched from a radiationsource, and a relative radiation intensity defined by a ratio of aradiation intensity measured by the spot sensor through an outer surfaceof the spot sensor over the radiation intensity dispatched from theradiation source.